The present invention relates to inorganic fiber-metal laminates and, more particularly, to a laminate formed from a ceramic fiber matrix bonded to a metal substrate with an inorganic binder and, even more particularly, to a ceramic fiber matrix-metal substrate laminate formed by using a very low weight percentage of inorganic binder to retain the original properties of the ceramic fiber matrix.
The high heat resistance and insulative properties of ceramic fibers have led to the development of many methods for making ceramic fiber insulation materials. Typically ceramic fibers, such as aluminosilicate, silica, alumina, zirconia, or glass fibers, are bound to each other by utilization of an inorganic binding agent such as silica, alumina, or zirconia. The binding agent is typically introduced to the fibers in the form of a colloidal dispersion, or sol, of the inorganic binding agent in a liquid dispersant. In addition, an organic material such as starch or metal fillers may be added to the binding agent, as taught by U.S. Pat. No. 4,041,199, issued to Cartwright. A ceramic fiber matrix is typically formed by confining a slurry of fiber and binder sol in the desired form. The binder sol is then gelled and dried to remove the liquid dispersant. Alternatively, the ceramic fibers are sometimes bound together mechanically.
The ceramic fiber matrix resulting from a conventional process exhibits good temperature resistance, but the utilization of the matrix as an insulation material is limited by the typically high content of inorganic binder and other fillers that must be utilized. The inorganic binder and fillers typically constitute between five and ninety percent by weight of the matrix. This high percentage of binder tends to stiffen the ceramic matrix, making it difficult to form around irregularly shaped objects, and increases the matrix density, making it less desirable for some applications, such as aircraft manufacture.
Conventional ceramic fiber matrices are generally capable of withstanding very high temperature, typically in excess of 1000.degree. C. However, these ceramic fiber matrices are not gas impermeable, precluding their use in some application, such as explosion proof or poisonous vapor-proof firewall construction. Further, if organic constituents have been added to the binding agent, conventional ceramic fiber matrices produce off-gases when heated to combustion temperatures.
Several methods have previously been developed for making fiber-metal composites to achieve gas impermeability and to facilitate installation of fiber insulation. One such laminate is disclosed by U.S. Pat. No. 4,395,453, issued to Linez, Jr. et al., which discloses the binding of cellulose fiber plies to aluminum foil plies, to form a heat-resistant laminate. An adhesive such as a silicate or polyethylene is used to bind the plies and produce a stiff laminate. The inclusion of metal plies in a fiber-metal laminate creates a heat-resistant barrier that is gas and liquid impermeable. However, the use of cellulose fibers limits the upper temperature range of the laminate. Off-gases would be produced by both the cellulose fiber and the polyethylene adhesive at elevated temperatures.
A flexible ceramic fiber-metal laminate has been disclosed by U.S. Pat. No. 4,447,345, issued to Kummermehr et al. A fiber matrix consisting of aluminosilicate fibers bound together by a microporous silica aerogel was laminated to a foil or sheet of metal. The silica aerogel represented between forty and ninety percent of the finished weight of the fiber matrix, greatly affecting the properties of the raw ceramic fiber utilized. In addition, a separate glue was required to attach the fiber matrix to the metal, potentially limiting the upper temperature range limit of the laminate.
These conventional fiber-metal laminates utilize thin sheets or foils of metal. Ceramic fiber-metal laminates utilizing thicker or irregularly shaped metal substrates have not previously been developed, restricting the ability to construct integral ceramic fiber insulated structural members.